1. Field of the Invention
This invention relates to an image signal processing apparatus for processing red, green, and blue color signals to supply yellow, magenta, and cyan signals used for color recording and an image formation apparatus including the same.
2. Description of the Prior Art
An image processing apparatus for processing R (red), G (green), and B (blue) color signals obtained from a color scanner, or the like to supply Y (yellow), M (magenta), and C (cyan) signals used for color recording is known and is used in a color printer or a color copy apparatus. FIG. 7 is a general block diagram of a prior art color recording apparatus having a prior art image signal processing apparatus. A digital color video data output unit 1 supplies digital color video signals R, G and B to an image signal processing apparatus 2 which processes the supplied digital color video signals R, G, and B and sends the processed digital color video signals to a color recording unit 3. The color recording unit 3 records the digital color video signals.
The digital color video signals supplied to the image signal processing apparatus 2 are R (red), G (green), and B (blue) signals indicative of luminances of R, G, and B components of the color video signal to be recorded, namely, the R, G, and B luminance data. These R, G, and B signals are converted into red, green, and blue density data by a density conversion unit 4. The red, green and blue density data are supplied to a color correction unit 5 which effects a masking processing. The color-corrected density data are subjected to a tone processing provided for compensating characteristics of the color recording unit 3 in a tone processing unit 6 and sent to the color recording unit 3.
If the color recording is carried out after converting the R, G, and B luminance data of three additive primary colors used in a color display or the like into three subtractive primary colors used in a color printer or the like through an equivalent correspondence therebetween, hue of the recorded color in the resultant color hard copy decreases. The reason why the decrease in hue is that colorants (toners) of cyan, magenta, and yellow have light absorbing characteristics in the other spectrum band than R, G, and B. For example, an ideal magenta colorant has an absorbing spectrum band at only green. In fact, magenta colorants show the absorbing characteristic considerably at spectrum bands of blue. The processing for compensating of this decrease in hue is referred to as a color correction which is effected by the color correction unit 5. A linear masking (one dimensionally masking) has been mainly used for such color correction. The linear masking is given by: ##EQU1## wherein Dr, Dg, Db are density data of red, green, and blue respectively; C, M, and Y are density data of cyan, magenta, and yellow colorants respectively; and T1 to T9 are matrix elements.
FIG. 8 is a block diagram of a prior art color correction unit 5 for effecting color correction represented by the EQ. (1). The density data Dr, Dg, and Db supplied from the density converting unit 4 is multiplied by the matrix elements T1 to T9 with multipliers 7A to 7I. Adders 8A to 8C add three results to provide density data of cyan, magenta, and yellow respectively.
The prior art linear masking mentioned above can be provided with a simple circuit structure. However, there are only nine parameters, so that there is a problem that the reproducing accuracy of color correction is not sufficient for the color printer.
Another prior art color correction employing a look-up table method with a large scale three-dimensional table provided for improving the reproduction quality of color through the color correction is known. The table requires a memory of 16 megabytes when the R, G, and B data are represented by eight bits respectively because the color correction is executed with three eight bit data, so that 2.sup.8 .times.2.sup.8 .times.2.sup.8 bits are required for each color and the total capacity of the memory is 48 megabytes. Therefore, there is problem that such large capacity memory increases the cost of the apparatus.